Simulation of Containment Atmosphere Stratification Experiment Using Local Instantaneous Description

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1 Intenational Confeence Nuclea Enegy fo New Euope 2004 Potoož Slovenia Septebe , fax PORT2004, Nuclea Society of Slovenia, Jaova 39, SI-1000 Ljubljana, Slovenia Siulation of Containent Atosphee Statification Expeient Using Local Instantaneous Desciption Mioslav Babić, Ivo Kljenak Reacto Engineeing Division Jožef Stefan Institute Jaova 39, SI-1000 Ljubljana, Slovenia ioslav.babic@ijs.si, ivo.kljenak@ijs.si ABSTRACT An expeient on ixing and statification in the atosphee of a nuclea powe plant containent at accident conditions was siulated with the CFD code CFX4.4. The oiginal expeient was pefoed in the TOSQAN expeiental facility. Siulated nonhoogeneous tepeatue, species concentation and velocity fields ae copaed to expeiental esults. 1 INTRODUCTION An ipotant topic in the field of nuclea safety is hydogen behavio in nuclea powe plant containent at accident conditions. One of the ain issues is to pedict whethe, due to atosphee ixing and statification, the local hydogen concentation in cetain pats of the containent will exceed flaability liits. Lately, any investigations about the possible application of so-called Coputational Fluid Dynaics (CFD) codes fo this pupose have been stated [1-5]. CFD codes solve the tanspot ass, oentu and enegy equations when a fluid syste is odeled using local instantaneous desciption. These investigations ae copleented by adequate expeients. Recently, the following integal expeiental facilities have been set up: TOSQAN, at the Institut de Radiopotection et de Sueté Nucléaie in Saclay (Fance), MISTRA, at the Coissaiat à l Enegie Atoique in Saclay (Fance), ThAI, at Becke Technologies GbH in Eschbon (Geany). In contast to olde expeiental facilities which wee used to investigate containent phenoena at accident conditions, the new facilities ae equiped with instuentation which allows easueent of local tepeatue, species concentation and velocities. Thus, the nonhoogeneous stuctue and the flow pattens of the containent atosphee ay be obseved, which enables a bette undestanding of ixing and statification pocesses in the containent of an actual nuclea powe plant. Besides, local expeiental easueents ay be used to assess the validity of siulations pefoed by CFD codes. In the pesent wok, the CFD code CFX4.4 [6], which is pefectly adequate fo the intended pupose, was used to siulate an expeient that was pefoed in the TOSQAN facility. Duing the test, stea, ai and heliu wee injected in a cylindical vessel at vaious 402.1

2 402.2 flow ates, and stea condensed on pats of the vessel walls. Calculated hoizontal pofiles of tepeatue, stea and heliu concentation, and vetical velocity at diffeent elevations in the vessel ae pesented and copaed to expeiental esults. 2 EXPERIMENT 2.1 Expeiental Facility The TOSQAN expeiental facility [7, 8] is a cylindical vessel with an intenal volue of (Fig. 1). The tepeatue of the vessel walls ay be contolled, so that the walls ae divided into a hot zone and a cold (condensation) zone. Stea and othe gases ae injected though a vetical tube located at the vessel cente-line. 2.2 Expeiental Pocedue In the pesent wok, a test pefoed within Phase 1 of the OECD Intenational Standad Poble on containent theal-hydaulics was consideed [7]. The vessel was initially filled with ai. Duing the test, ai, stea and heliu wee injected inteittently with vaious ass flow ates. The tepeatue of the walls was contolled. The sup was dained continuously. Stea condensation occued only on walls with lowe tepeatue (aea: ). The theal-hydaulic behavio was deteined by the following physical phenoena: gas injection, stea condensation, heat tansfe and buoyant flow. Duing cetain phases of the expeient, steady states wee eached when the stea condensation ate becae equal to the stea injection ate, with all bounday conditions eaining constant. Thee steady states (efeed in the pesent wok as 1, 2 and 3), which wee obtained with diffeent bounday conditions, wee siulated independently. The (easued) paaetes of the steady states ae povided in Table 1 [8]. The tepeatues of the uppe and lowe non-condensing wall wee ºC and ºC, espectively. The tepeatue of the condensing wall was ºC duing steady states 1 and 3, and 107.8ºC duing steady state 2. Table 1. Paaetes of steady states duing expeient in TOSQAN vessel Steady state 1 Steady state 2 Steady state 3 Pessue, ba Aveage tep., ºC Ai ass, kg Initial stea ass, kg Not epoted Heliu ass, kg Stea injection ate, g/s INPUT MODEL 3.1 Geoetic Model A two-diensional axisyetic odel (in cylindical coodinates) fo the code CFX4.4 was developed. The coputational doain is a block with 160 cells in the axial (vetical) diection and 30 cells in the adial (hoizontal) diection (Fig. 2). Thee ae 4800 Poceedings of the Intenational Confeence Nuclea Enegy fo New Euope, Potoož, Slovenia, Sept. 6-9, 2004

3 402.3 cells altogethe. The width (adial diection) of each cell depends on the axial location. Thus, cells located within the vessel sup ae uch naowe than cells located within the condensation egion (Fig. 1). The condensation zone extends fo z = to z = and includes 67 ows of cells. In the condensation zone, the width of the cells contiguous to the vessel wall is 2.0 c. TOSQAN vessel Condensation zone Injection tube sup Figue 1. Scheatic view of the TOSQAN expeiental facility [7]. Figue 2. Nueical gid of TOSQAN vessel fo CFX code. 3.2 Physical Model The ai-stea and ai-stea-heliu atosphees wee teated as single-phase gaseous ixtues, which ae hoogeneous within each coputational cell. The following options wee pescibed in the CFX coand file : copessible flow, tubulent flow (k-ε odel), buoyant flow, no-slip condition at the vessel wall. The default options of the CFX4.4 code, which coespond to these physical odels, wee applied. Poceedings of the Intenational Confeence Nuclea Enegy fo New Euope, Potoož, Slovenia, Sept. 6-9, 2004

4 402.4 The odeling of condensation is not ipleented in ost cuent CFD codes, including CFX. In the pesent wok, stea condensation was odeled as a sink of ass and enthalpy by applying the Uchida coelation [9] that was basically developed fo an integal appoach. The condensate fil on the cold wall was not consideed. The Uchida coelation is based on expeients on natual convection fo elatively sall vetical plates and was oiginally developed to be used fo integal (volue-aveaged) odeling of stea condensation in the pesence of non-condensable gases. Basically, the stea condensation ate on a vessel wall with aea A is obtained fo the expession: M 0 = C u (ρ stea /ρ non-cond ) 0.8 A (T T wall ) / h lg (1) whee C u is an adjustable coefficient, h lg denotes the latent heat of evapoation, and all the physical vaiables, except T wall, efe to bulk flow conditions in the vessel. The coesponding enthalpy sink (enthalpy flow) due to condensation ay then be calculated as: H 0 = 0 c p (T T ef ) (2) whee T ef denotes soe efeence tepeatue at which the enthalpy is zeo. We have decided to use Uchida s coelation, although it is known to be in eo when condensing sufaces ae tall so that condensate fils becoe thick and ceate significant theal esistance, and when stea jets and othe souces of foced convection augent natual convection flow [10]. The odeling of condensation was ipleented in a use-defined suboutine. Sinks of ass and enthalpy occued in cells contiguous to the condensing wall. In each cell, the ass sink was calculated fo Eq. (1) whee the bulk flow physical quantities (tepeatue, stea density, ai density) wee evaluated at the cell cente. Fo each cell, the aea A was calculated as: A = 2π R z (3) whee R denotes the local vessel adius and z the cell height. Siilaly, the enthalpy sink was calculated fo Eq. (2). The sensible heat flux though vessel walls was coputed by the CFX code. 4 RESULTS AND DISCUSSION Each steady state was siulated sepaately. Fo each siulation, the initial pessue and aveage tepeatue in the vessel wee set to the easued values. The value of the coefficient C u (Eq. 1) was adjusted to obtain a easonably good ageeent between easued and calculated pessue and aveage atosphee tepeatue duing steady states. The sae value C u = 200 W/ 2 K was used fo all thee steady states. The initial stea ass faction was calculated so that the ass of ai in the vessel was equal to the su of the initial and injected asses. In the siulation of steady state 3, the ass of injected heliu pio to the steady state was also taken into account. Fo each steady state, the siulation was un until the condensation ate becae equal to the stea injection ass flow ate, and the vaiations of pessue and aveage tepeatue becae sall enough. Measued [8] and calculated pessue and aveage tepeatues, obtained with the pescibed value of the coefficient C u, ae pesented in Table 2. The diffeences between expeiental and siulated values ae elatively sall, except pehaps fo the pessue duing steady state 2 (whee the elative diffeence is still lowe than 5%). Poceedings of the Intenational Confeence Nuclea Enegy fo New Euope, Potoož, Slovenia, Sept. 6-9, 2004

5 402.5 Table 2. Measued and calculated pessue and aveage tepeatue. Pessue, ba Steady state 1 Steady state 2 Steady state 3 Expeient Siulation Aveage Expeient tepeatue, ºC Siulation Figues 3-12 show the easued and calculated adial pofiles of tepeatue, stea and heliu voluetic concentation and vetical velocity duing the thee steady states at epesentative vetical positions (elevations). Although the coputational doain was only one half of a vetical plane, syetic esults ae shown ove the entie vessel fo copaison with expeiental data on both sides of the vessel axis. The uncetainties of tepeatue, voluetic concentation and velocity easueents ae ±1 ºC, up to ± 2.3% and up to ±10 % of the easued value, espectively. Fo the tepeatue pofiles, a vey good ageeent was obtained fo steady state 1 (Fig. 3) and steady state 3 (Fig. 9). Fo steady state 2 (Fig. 6), the siulated tepeatues at highe elevations ae soewhat undepedicted. In paticula, the code did not siulate the high tepeatue at the vessel cente-line above the injection tube. Fo the concentation pofiles, the siulation eplicated the elatively hoogeneous atosphee duing steady states 1 and 3 (Figs. 4, 10 and 11). The appaently lage discepancies ae due to the scale on the odinata axis. Fo steady state 2, whee the atosphee was less hoogeneous due to the highe stea injection ate, a good ageeent ay be obseved at the highest and lowest elevation (except in the nea-wall egion, which was not odeled in detail in the pesent wok). Fo the inteediate elevation, the code eplicated the expeiental patten but with a shift towads highe values. The vetical velocities wee well siulated fo steady states 1 and 3 (Figs. 5 and 12) although in the fist case, the velocity in the vessel coe was a little ovepedicted. Fo steady state 2 (Fig. 8), the code did not siulate the elatively high velocity at the vessel cente-line above the injection tube. o C T Volue faction Figue 3. Steady state 1: expeiental and siulated tepeatue adial pofiles. Figue 4. Steady state 1: expeiental and siulated adial pofiles of stea volue faction. Poceedings of the Intenational Confeence Nuclea Enegy fo New Euope, Potoož, Slovenia, Sept. 6-9, 2004

6 402.6 u /s o C T Figue 5. Steady state 1: expeiental and siulated adial pofiles of vetical velocity. Figue 6. Steady state 2: expeiental and siulated tepeatue adial pofiles. Volue faction u /s Figue 7. Steady state 2: expeiental and siulated adial pofiles of stea volue faction. Figue 8. Steady state 2: expeiental and siulated adial pofiles of vetical velocity. o C T Volue faction Figue 9. Steady state 3: expeiental and siulated tepeatue adial pofiles. Figue 10. Steady state 3: expeiental and siulated adial pofiles of stea volue faction. Poceedings of the Intenational Confeence Nuclea Enegy fo New Euope, Potoož, Slovenia, Sept. 6-9, 2004

7 402.7 Volue faction /s u Figue 11. Steady state 3: expeiental and siulated adial pofiles of heliu volue faction. Figue 12. Steady state 3: expeiental and siulated adial pofiles of vetical velocity. Figues illustate the stea and heliu (fo steady state 3) siulated concentation pattens in the vessel. On Fig. 14, the influence of the highe stea injection ate ay be obseved. Figue 16 shows that, duing steady state 3, heliu is alost hoogeneously distibuted in the vessel (except fo a sall egion nea the injection tube). Figue 13. Steady state 1: stea voluetic concentation. Figue 14. Steady state 2: stea voluetic concentation. Figue 15. Steady state 3: stea voluetic concentation. Figue 16. Steady state 3: heliu voluetic concentation. Poceedings of the Intenational Confeence Nuclea Enegy fo New Euope, Potoož, Slovenia, Sept. 6-9, 2004

8 CONCLUSIONS An expeient on containent atosphee statification at accident conditions, pefoed in the TOSQAN expeiental facility, was siulated with the CFD code CFX4.4. A two-diensional axisyetic odel was developed, and stea condensation was odeled by ipleenting an integal coelation in the CFD code. Despite soe discepancies, the ageeent between expeiental and calculated esults shows that the poposed appoach is adequate and could be applied to siila siulations. ACKNOWLEDGMENTS The eseach pesented in this wok was pefoed within the eseach poject J (contact no ) financed by the Ministy fo Education, Science and Spots of the Republic of Slovenia. The Jozef Stefan Institute is a ebe of the Sevee Accident Reseach Netwok of Excellence (SARNET) within the 6th EU Faewok Pogae. The benefit fo Euopean Coission RTD Pogae fo eseach is acknowledged. REFERENCES [1] B.L. Sith, M. Milelli, S. Shepel, Aspects of nuclea eacto siulation equiing the use of advanced CFD odels, IAEA-OECD Meeting on Use of CFD Codes fo Safety Analysis of Reacto Systes including Containent, Pisa, Italy, [2] J.A. Lycklaa à Nijeholt, J. Hat, CFD siulation of theal-hydaulics in the PHEBUS FP containent vessel, Poc. 8th Int.Conf. Nuclea Engineeing ICONE8, Baltioe, USA, [3] J.M. Matin-Valdepenas, M.A. Jienez, CFD analyses of hydogen isk within PWR containents, IAEA-OECD Meeting on Use of CFD Codes fo Safety Analysis of Reacto Systes including Containent, Pisa, Italy, [4] A. Rastogi, CFD application to eacto safety pobles with coplex flow egies, IAEA-OECD Meeting on Use of CFD Codes fo Safety Analysis of Reacto Systes including Containent, Pisa, Italy, [5] N.B. Siccaa, M. Houkea, E.M.J. Koen, CFD analyses of stea and hydogen distibution in a nuclea powe plant, IAEA-OECD Meeting on Use of CFD Codes fo Safety Analysis of Reacto Systes including Containent, Pisa, Italy, [6] AEA TECHNOLOGY plc, CFX-4.3: Solve Manual, Hawell, United Kingdo, [7] P. Conet, J. Malet, E. Pocheon, TOSQAN Expeiental Results of the Ai-Stea Phase, OECD Int. Stand. Pobl. Containent Theal-Hydaulics ISP-47, IRSN, Saclay, Fance, [8] Ph.Bun, P.Conet, J.Malet, B.Menet, E.Pocheon, J.Vendel, M.Caon-Chales, J.J.Quillico, H.Paillee, E.Stude, Specification of Int. Stand. Pob. Containent Theal- Hydaulics ISP-47, Step 1: TOSQAN MISTRA, IRSN and CEA, Saclay, Fance, [9] T.L. Geoge, A. Singh, Sepaate Effects Tests fo GOTHIC Condensation and Evapoative Heat Tansfe Models, Nuclea Engng Design 166, 1996, pp [10] P.F. Peteson, Theoetical Basis fo the Uchida Coelation fo Condensation in Reacto Containents, Nuclea Engng Design 162, 1996, pp Poceedings of the Intenational Confeence Nuclea Enegy fo New Euope, Potoož, Slovenia, Sept. 6-9, 2004